Rotavirus (RV) is one of the causative agents of acute diarrhea in foals and calves. The mode of RV transmission among animals is still unclear, although RV has been reported to be wide-spreading in the animal populations. The present study is concerned with molecular virologic and immunologic features of RV infection in the animals. (1) A series of equine RVs was isolated from diarrheal foals in a light-horse farm from 1986 to 91. All the represented RVs in each year showed similar genomic RNA and serotype G3, suggesting the persistence of a particular RV strain in this farm for long time. In 1987, however, one isolate (CH3) showed a unique electropherotype (EPT) of viral RNA which differed from the others that widely prevailed on this farm. The VP7 amino acid sequence of strain CH3 also showed a high homology to that of G14 strain FI23, and the VP4 sequence, similar to that of strain H2, respectively. (2) Foal diarrhea with G3 RV broke out on the above-mentioned farm. In 1989, all of the foals received bovine colostral immunoglobulin (Ig) powder during the epidemic, and their morbidity was lower than that observed in 1987, when the Ig powder was not administered to foals. This suggests that the administration of Ig powder might partially prevent foal diarrhea. (3) Eight RV strains were isolated from fecal samples in epidemic diarrhea of new-born calves. Viral RNAs of the strains were classified into 5 EPTs. However, some strains presented not a single but mixed-up conformations of dual or triple EPTs. These data suggest that the mixed infection of distinct RVs in one calf cause the rearrengement of genomic segments among the strains. (4) In a closed dairy farm, bovine RV was isolated from 35 (16%) of 222 fecal specimens repeatedly collected from 59 calves born during the years 1990-93. The affected calves had IgA and IgG coproantibody levels much higher than those of normal calves 3 weeks old (P<0.05). Most of the 35 strains were isolated from calves aged 2 to 6